It was the beginning of the end of World War II: Wave upon wave of allied troops storming the coast of northern France to liberate the country and its people from Nazi occupation, in the largest seaborne invasion in history.

Braving everything from heavy seas to machine gun fire and grenades, soldiers clambered ashore at beaches codenamed Utah, Omaha, Gold, Juno and Sword and began to fight their way inland.

Thousands died in the attempt; today their bodies fill military cemeteries along the Normandy coast.

Some of their former comrades make the trip back to France every year.

D-Day: Russia's sacrifice

The Allied invasion of Normandy – American troops storm the beaches of Normandy, France, on June 6, 1944. D-Day was the largest seaborne invasion in history. More than 150,000 Allied troops -- about half of them Americans -- invaded Western Europe, overwhelming German forces in an operation that proved to be a turning point in World War II.

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The Allied invasion of Normandy – U.S. troops wait to disembark a landing craft on D-Day.

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The Allied invasion of Normandy – American troops help their injured comrades after their landing craft was fired upon. Allied forces secured the beaches at a cost of about 10,000 casualties.

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The Allied invasion of Normandy – French commandos equipped with bicycles disembark from their landing craft after Allied forces stormed the Normandy beaches. Germans rained mortars and artillery down on Allied troops, killing many before they could even get out of their boats. Fighting was especially fierce at Omaha Beach, where Nazi fighters nearly wiped out the first wave of invading forces and left the survivors struggling for cover.

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The Allied invasion of Normandy – The British Army's 50th Infantry Division lands on beaches in Normandy. This photograph is part of an exhibit in London at the Imperial War Museum.

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The Allied invasion of Normandy – An aerial view shows 32 intentionally sunk American merchant ships that served as a breakwater as well as anti-aircraft platforms.

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The Allied invasion of Normandy – Traffic builds up on the beach and on the road leading to Le Hamel, France, during the landing of the 50th Infantry Division.

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The Allied invasion of Normandy – U.S. troops and vehicles are ready to disembark. D-Day was one of history's most consequential and gut-wrenching battles.

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The Allied invasion of Normandy – A sign outside of Trinity Church in New York invites worshippers to pray for Allied victory in the D-Day invasion.

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The Allied invasion of Normandy – Commandos with the Royal Navy's 4th Special Service Brigade advance to Juno Beach at Saint-Aubin-sur-mer.

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The Allied invasion of Normandy – Lance Cpl. Walter Ray of the British Army shares a bottle of rum he found floating in the sea.

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The Allied invasion of Normandy – U.S. Gen. Dwight D. Eisenhower, left, supreme commander of the Allied forces, and British Field Marshal Bernard Montgomery, right, discuss plans at an undisclosed location in June 1944. The Allies went to elaborate lengths to maintain secrecy and mislead Adolf Hitler. They employed double agents and used decoy tanks and phony bases in England to hide actual troop movements.

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The Allied invasion of Normandy – Eisenhower gives the order of the day to paratroopers in England. "Full victory -- nothing else" was the command just before they boarded their planes to participate in the first wave. The invasion -- code-named Operation Overlord -- had been brewing for more than two years.

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The Allied invasion of Normandy – British troops pose for a photograph with a French woman in La Breche d'Hermanville.

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The Allied invasion of Normandy – U.S. troops huddle behind the protective front of their landing craft as it nears a beachhead. Smoke in the background is naval gunfire, giving cover to troops on land.

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EXPAND GALLERY

For Harold Bradley, there is an added reason why landing at Sword Beach is something he could never forget: It was his 21st birthday.

"I landed in Normandy on my 21st," he says. "I remember saying to a friend -- it's my birthday today Rob, and he said: 'It seems a pretty good day to have it.' But of course once you are on those boats you aren't thinking about birthdays."

A gunner with Britain's Royal Artillery, Bradley was there to support troops who had captured the beach three days earlier.

"We were going to join the division that landed on D-Day which required more support from guns, and that was our job. We landed on the 9th, we were there in support of the infantry division; within about an hour of landing we were firing guns."

Bradley lost some of his closest friends in the fighting that followed. Just weeks before the end of the conflict, he was captured by the Germans -- his family were told he was missing in action, and presumed dead -- before being freed by American soldiers.

"I have often wondered since, many times, how my mother must have felt having a telegram from the Foreign Office saying [I was] missing, believed killed, and then a voice comes on the phone and says 'Hello Mum,'" he says. "It must have been stunning."

Seventy years on from D-Day, preparations have been in full swing for months: New flags and banners have gone up, the grass at the cemeteries has been neatly trimmed, and everything has been given a spit and polish ahead of the anniversary visit by world leaders including Queen Elizabeth II and U.S. President Barack Obama.

Michel Joyeux from Normandy was planning to plant red, white and blue flowers in front of his house when CNN visited -- his flags were already flying proudly.

"With me they are up every year," he said. "It is an honor for me to decorate and commemorate all the soldiers who died."

The cathedral at Bayeux has a new bronze bell to mark the anniversary. Local priest, Father Laurent Berthout, said it celebrated "peace and reconciliation between the different countries, and between France and Germany, and the friendship between the United States and France and England."

But Berthout said not all of those in Normandy have completely positive memories of the liberation -- some can't forget how destructive it was.

Charles Moncouteau, 87, told CNN his village never joins in the commemorations, because it was almost completely destroyed by the fighting.

This year, French President Francois Hollande has ordered that attention be focused on the civilian losses as well as the military ones; it is estimated that, during the Normandy campaign alone, 20,000 French civilians lost their lives.

Three-quarters of Caen, where Hollande and more than a dozen heads of state and government will gather this week, was destroyed by the fighting when German troops held out there in the months following D-Day, prompting the Allies to lay siege to the city.

Scars left by the conflict remain, 70 years later.

But many of today's residents of Normandy are simply grateful for the actions of those soldiers -- in some cases, decades before they were even born.

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Remembering D-Day

"My gratitude is huge," says law student Paul Guillotte. "I am blessing every day the heroes who came to rescue us."

On June 6, others will say thank you in their own way.

American jazz pianist Bill Carrothers will perform alongside more than 100 teenagers from schools across the U.S. and France, offering a musical tribute to those who fought in World War II.

"It's a tiny and fairly insignificant way of giving back to a lot of veterans who went before us and died, fighting for an idea, a very important idea that I think some of the modern wars somewhat lost," he explained.

"The landings in Normandy were an important moral event. It wasn't a war for oil, prestige or power. It was for an idea -- maybe the last time in our history that I can think of that it has been the case."

For decades since the war's end, Harold Bradley has declined to take part in events commemorating D-Day.

"It isn't me," he explains. "It's like wearing medals, you know. I don't want to walk up and down with medals on saying: 'look at me.'"

But this year, on the 70th anniversary of the decisive battle, Bradley will be in Normandy to pay tribute to those who fell, alongside other elderly veterans and world leaders.

"I am extremely fortunate to be as fit as well as I am, but obviously it can't last forever -- [I'm] 91 on June 9," he says. "In so much as it could well be the last time, I shall go."

As one of the last remaining soldiers who can recall those bloody days in June 1944, he hopes that such battles remain firmly in the past.

"It would be a dream to say stop wars but at least reduce them, because at the end of a war what have you got? No-one's won anything. Millions dead and you still have the same problem."